Magnetostrictive torque sensor

ABSTRACT

A torque sensor has a magnetostrictive member fixed to a shaft. The magnetostrictive member is strained by the torque applied to the shaft. Exciting coils generate flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. Detecting coils detect the flux variation. The exciting coils and the detecting coils are wound about bobbins. A stator supports the bobbins such that the coils surround and face the magnetostrictive member. The stator is rotatably supported relative to the shaft and is formed with stator pieces. The stator has annular grooves formed in its inner surface to accommodate the bobbins. One end of each bobbin is fixed to one wall of the corresponding annular groove. A clearance is formed between the outer end of the bobbin and the other wall of the annular groove, and another clearance is formed between the outer circumferential surface of the bobbin and the inner circumferential wall of the annular groove. The clearances accommodate thermal expansion of the bobbins, which improves the performance of the torque sensor.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetostrictive torque sensor thatdetects torque acting on a shaft by sensing magnetostrictive changes.

For example, Japanese Unexamined Patent Publication No. 5-118938 andJapanese Unexamined Patent Publication No. 59-77326 disclose suchmagnetostrictive torque sensors. A magnetostrictive torque sensorincludes a shaft, a magnetostrictive member fitted about the shaft anddetecting coil. Torque applied to the shaft deforms the magnetostrictivemember, which changes the magnetic permeability of the magnetostrictivemember. This varies the flux, or inductance, of the magnetostrictivemember. Changes in the inductance are detected based on electromotiveforce induced in the detecting coil. The torque applied to the shaft isdetected accordingly.

As illustrated in FIG. 4, a prior art magnetostrictive torque sensorincludes a housing 30, a shaft 32 rotatably supported by the housing 30,a sleeve 33 fitted about the shaft 32 and a magnetostrictive member 34fitted about the sleeve 33. A cylindrical stator 35 is located about themagnetostrictive member 34. The stator 35 is supported on the shaft 32by bearings 38. The bearings 38 allow the stator 35 rotate relative tothe shaft 32. The stator 35 includes a cylindrical yoke 36 and a pair ofretainers 37 for supporting the yoke 36. The yoke 36 has a pair of resinbobbins B. An exciting coil 39 and a detecting coil 40 are wound abouteach bobbin B. The exciting coils 39 and the detecting coils 40 surroundand face the surface of the magnetostrictive member 34. When analternating current is supplied to the exciting coils 39, the excitingcoils 39 generate flux. The flux forms a magnetic circuit through theyoke 36 and the magnetostrictive member 34.

The surface of the magnetostrictive member 34 is divided into twodetection regions X and Y. Grooves 34 a are formed in each region X, Y.The grooves 34 a in the region X are inclined by forty-five degreesrelative to the axis of the shaft 32, and the grooves 34 a in the regionY are inclined by minus forty-five degrees relative to the axis. Whentorque is applied to the shaft 32, a compressive force acts on one ofthe detection regions X and Y and a tensile force acts on the otherregion depending on the rotational direction of the shaft 32. A tensileforce increases the magnetic permeability of the detection regions X, Yand a compression force decreases the magnetic permeability of theregions X, Y. Variations in the magnetic permeability of themagnetostrictive member 34 change the voltages induced in the detectingcoils 40. That is, the detecting coils 40 send varying voltage values toa processor (not shown). The processor computes the torque applied tothe shaft 32 based on the voltage values from the detecting coils 40.

The yoke 36 is located close to the magnetostrictive member 34 toimprove the sensitivity of the torque sensor. To facilitate theinstallation of the bobbins B (the coils 39, 40) the yoke 36 includestwo semi-cylindrical pieces 41, 42, as illustrated in FIG. 5. Wheninstalling the coils 39, 40, the bobbins B, about which the coils 39, 40are wound, are attached to the inner wall of the yoke 36. Thereafter,the pieces 41, 42 are secured to each other along two joints 43 withnon-conductive adhesive. Since the yoke 36 is divided into two piecesalong a plane that includes the axis and since the non-conductiveadhesive is located between the pieces 41, 42, eddy currents in thecircumferential direction of the yoke 36 are blocked by the joints 43.The eddy currents, which would otherwise adversely affect thesensitivity of the torque sensor, are reduced. This improves thesensitivity of the torque sensor. However, since the magnetic reluctanceof the joints 43 is much greater than that of the other parts, themagnetic symmetry about the axis of the yoke 36 is impaired. The loweredmagnetic symmetry of the yoke 36 causes the detection voltage of thetorque sensor to change in accordance with the rotational position ofthe shaft 32. Therefore, even if there is no torque acting on the shaft32, the torque sensor may erroneously detect that a torque is acting onthe shaft 32.

Torque deforms the magnetostrictive member 34, and deformation of themember 34 changes the inductance of the coils. The torque sensor detectstorque based on the changes of the inductance. However, the inductancealso changes when the distance between the magnetostrictive member 34and the coils 39, 40 changes. When the distance between themagnetostrictive member 34 and the coils 39, 40 changes, the torquesensor falsely detects torque even if there is no torque acting on theshaft 32. Therefore, it is imperative that the distance between themember 34 and the coils 39, 40 be constant to guarantee the precision ofthe torque sensor.

As described above, the stator 35 includes the yoke 36 and the tworetainers 37, shown in FIG. 4, and the yoke 36 includes twosemi-cylindrical pieces 41, 42, shown in FIG. 5. Due to dimensionalerrors created during assembly, the cross-section of the stator 35 isnot completely round. That is, when assembling the pieces 41 and 42 toform the yoke 36, their relative positions may vary. Also, the positionsof the retainers 37 may vary relative to the yoke 36. The variation ofthe relative positions of the parts prevents the inner surface of theretainer 35 from forming a perfect circle about the axis of the shaft32. Further, the edges of the pieces 41, 42 are exposed on the innersurface of the yoke 36, which disturbs the distribution of the flux.

The resin bobbins B are press fitted in the metal yoke 36. The bobbins Bhave a greater coefficient of thermal expansion than that of the yoke36. Therefore, when the temperature of the sensor is relatively high, astress F is generated between each bobbin B and the yoke 36. The stressF displaces the positions of the exciting coils 39 and the detectingcoils 40. Therefore, even if there is no torque acting on the shaft 32,the electromotive force induced in the detecting coils 40 is changed. Inother words, the magnetic characteristics of the coils 39, 40 areaffected by temperature changes. This degrades the temperaturecharacteristics of the torque sensor thereby lowering the detectionaccuracy of the torque sensor.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide atorque sensor that accurately detects torque despite temperaturechanges.

To achieve the foregoing and other objectives and in accordance with thepurpose of the present invention, a torque sensor for sensing torqueapplied to a shaft is provided. The sensor includes a magnetostrictivemember, an exciting coil, a detecting coil, a bobbin and stator. Themagnetostrictive member is fixed to the shaft is strained by the torqueapplied to the shaft. The exciting coil generates flux running throughthe magnetostrictive member. The flux varies in accordance with thestrain of the magnetostrictive member. The detecting coil detects theflux variation. The exciting coil and the detecting coil are wound aboutthe bobbin. The stator supports the bobbin such that the coils surroundthe magnetostrictive member. The stator is rotatably supported relativeto the shaft and is formed with a plurality of stator pieces. The statorhas an annular groove formed in its inner surface to accommodate w thebobbin. The annular groove has a first end wall, a second end wall and acircumferential wall. The bobbin further has a first end surface, asecond end surface and a circumferential surface. The first end surfaceof the bobbin is fixed to the first end wall of the annular groove. Afirst annular clearance is formed between the circumferential surface ofthe bobbin and the circumferential wall of the annular groove. A secondannular clearance is formed between the second end surface of the bobbinand the second end wall of the annular groove.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a cross-sectional view illustrating a torque sensor accordingto a first embodiment of the present invention;

FIG. 2 is an enlarged partial cross-sectional view illustrating thetorque sensor of FIG. 1;

FIG. 3 is an enlarged cross-sectional view illustrating a torque sensoraccording to a second embodiment of the present invention;

FIG. 4 is a cross-sectional view illustrating a prior art torque sensor;

FIG. 5 is a perspective view illustrating the yoke of the torque sensorshown in FIG. 4;

FIG. 6 is a perspective view illustrating a stator according to thepresent invention;

FIG. 7 is an enlarged cross-sectional view illustrating a torque sensoraccording to a third embodiment of the present invention; and

FIG. 7A is an enlargement of the portion of FIG. 7 encircled by a brokenline.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A torque 1 sensor according to a first embodiment of the presentinvention will now be described with reference to FIGS. 1, 2 and 6.

As shown in FIG. 1, the torque sensor 1 includes a detection member 3and a detector 4. The detection member 3 is fitted about a shaft 2 andthe detector 4 detects magnetic changes due to strain of the member 3. Apair of radial bearings 6 allow the detection member 3 and the detector4 to rotate relative to each other.

The detection member 3 is welded to the shaft 2 and includes acylindrical magnetostrictive member 5, sleeves 5 b adjacent to themember 5 and threaded portions 5 c formed at the ends. Themagnetostrictive member 5 has a magnetostrictive property. Themagnetostrictive member 5 can be made of a soft magnetic material havinga high magnetic permeability such as permalloy and iron-nickel-chromiumalloy. The magnetostrictive member 5 also can be made ofmagnetostrictive material including iron and aluminum or an amorphousmagnetostrictive material. The magnetostrictive member 5 includes tworegions X and Y, in which grooves 5 a are formed. The grooves 5 a areequally spaced apart in the circumferential direction. The grooves 5 ain one of the regions X are inclined by forty-five degrees relative tothe axis, and the grooves 5 a in the other region Y are inclined byminus forty-five degrees relative to the axis. The bearings 6 are pressfitted between the sleeves 5 b and the detector 4.

The detector 4 includes a stator 7 and bobbins B1, B2. The stator 7 issupported on the detection member 3 by a pair of bearings 6. An excitingcoil 8 and a detecting coil 10 are wound about the bobbin B1, and anexciting coil 9 and a detecting coil 11 are wound about the bobbin B2.The exciting coils 8, 9 are located inside the detecting coils 10, 11.The stator 7 includes an annular inner yoke 14, which is press fitted toand couples a pair of outer annular yokes 12 and 13. That is, the stator7 is formed by the first outer annular yoke 12, the inner annular yoke14 and the second outer annular yoke 13, which are connected end-to-endin series in the axial direction of the shaft 2. The outer yokes 12, 13and the inner yoke 14 are made of metal such as permalloy and form twocircumferential grooves 24, 25. The bobbins B1, B2 are located in thegrooves 24, 25 and face the detection regions X, Y, respectively. Thebobbins B1, B2 are made of a resin such as polybutylene terephthalate(PBT).

The supporting structure of the bobbins B1, B2 will now be describedwith reference to FIG. 2. Since the yokes 12, 13 and the bobbins B1, B2of the detector 4 are identical, only the left yoke 12 and the bobbin B1will be described. The first outer yoke 12 includes an end portion 12 aand a stopper portion 12 b. The end portion 12 a is located at the outerend of the first outer yoke 12. The inner diameter of the stopperportion 12 b is smaller than that of the end portion 12 a. Therefore, astep is defined between the end portion 12 a and the stopper portion 12b. The bearing 6 is fitted in the end portion 12 a and contacts thestep. The first outer yoke 12 also has an intermediate portion 12 clocated axially inward of the stopper portion 12 b. The inner diameterof the intermediate portion 12 c is smaller than that of the stopperportion 12 b. The inner diameter of the end portion 12 a issubstantially the same as the outer diameter of the bearing 6, such thatthe outer ring of the bearing 6 is lightly press fitted in the endportion 12 a.

The distance between the inner surface of the intermediate portion 12 cand the detection region X of the magnetostrictive member 5 isdetermined such that the sensitivity of the torque sensor is maximized.

The first outer yoke 12 also includes a holder portion 12 d and an innerend portion 12 e. The holder portion 12 d holds the bobbin B1 and theinner end portion 12 e contacts the yoke ring 14. The inner diameter ofthe holder portion 12 d is slightly larger than the outer diameter ofthe bobbin B1. The axial length L1 of the holder portion 12 d is longerthan the axial length L2 of the bobbin B1. Therefore, a clearance Cl isdefined between the inner surface of the holder portion 12 d and theouter surface of the bobbin B1. Also, a clearance C2 is defined betweenthe right end surface of the bobbin B1 and the left end surface of theinner yoke 14. Specifically, the radial and axial dimensions of theclearances C1 and C2, respectively, between 0.1 mm and 0.2 mm. Thecoefficient of thermal expansion of the bobbins B1, B2 is greater thanthat of the yokes 12, 13 and the yoke ring 14. The clearances C1, C2accommodate thermal expansion of the bobbins B1, B2.

An annular projection 15 is formed on the side wall of the holderportion 12 d, and an annular recess 16 is formed in the outer side wallof the bobbin B1. Specifically, the recess 16 is located at the radiallyinner part of the bobbin B1, as shown in FIG. 2. The projection 15engages with the recess 16, which facilitates accurate positioning ofthe bobbin B1 and maintains the clearance C1 between the inner surfaceof the holder portion 12 d and the outer surface of the bobbin B1.

The inner diameter of the inner end portion 12 e of the first outer yoke12 is larger than the outer diameter of the bobbin B1, which allows thebobbin B1 to be inserted into the holder portion 12 d from the inner endportion 12 e. The outer diameter of the yoke ring 14 is larger than thatof the bobbin B1 and is substantially the same as the inner diameter ofthe inner end portion 12 e. The inner diameter of the inner yoke 14 issubstantially the same as that of the bobbin B1.

The manufacture of the stator 7 will now be described. First, adhesiveis applied on the surfaces 100, 101 of the projection 15 of the firstouter yoke 12. Then, the bobbin B1 is inserted in the holder portion 12d of the yoke 12 until the recess 16 engages with the projection 15. Thebobbin B1 is thus fixed to the surfaces 100, 101. The adhesive on thesurface 100 prevents the bobbin B1 from moving radially, and theadhesive on the surface 101 prevents the bobbin B1 from moving axially.Thereafter, the yoke ring 14 is lightly press fitted into the inner endportion 12 e.

As illustrated, the holder portion 12 d of the first outer yoke 12 andthe side wall of the inner yoke 14 define the circumferential groove 24to accommodate the bobbin B1. In the same manner, the bobbin B2 isaccommodated in the circumferential groove 25 defined by the holderportion 13d of the second outer yoke 13 and the other side wall of theinner yoke 14.

As shown in FIG. 1, fasteners 17, 18 are engaged with the threadedportions 5 c to determine the axial position of the torque sensor 1. Thefasteners 17, 18 press the stator 7 through the bearings 6 therebypreventing the stator 7 from moving axially.

The exciting coils 8, 9 are connected to an alternating-current powersupply (not shown). The detecting coils 10, 11 are connected to aconventional processor (not shown). When an alternating-current issupplied to the exciting coils 8, 9, the exciting coils 8, 9 produceflux. The flux forms two magnetic circuits through the stator 7 and themagnetostrictive member 5. The flux produced by the exciting coil 8extends along the grooves 5 a in the region X, and the flux produced bythe exciting coil 9 extends along the grooves 5 a in the region Y.Changes of the flux induce electromotive force in the detecting coils10, 11. The induced electromotive force is sent to the processor.

When torque acts on the shaft 2, one of the detection regions X, Yreceives a compression force and the other region receives a tensileforce depending on the direction of the torque. A tensile forceincreases the magnetic permeability of the magnetostrictive member 5 anda compression force decreases the magnetic permeability of themagnetostrictive member 5. Each detecting coil 10, 11 produces aninduced electromotive force representing either a compression force orby tensile force. Specifically, when a region X or Y receives a tensileforce, the coil 10 or 11 corresponding to the region generatesrelatively great electromotive force. When a region X or Y receives acompression force, the coil 10 or 11 corresponding to the regiongenerates a relatively small electromotive force. In short, thedetecting coils 10, 11 send different voltages to the processor.

The processor includes a differential circuit, a rectifier circuit and atorque computing circuit (none of which is shown). The processorreceives voltages from the detecting coils 10, 11. The differentialcircuit computes the difference between the voltages. The rectifiercircuit rectifies the computed difference and outputs the rectifiedsignal to the torque computing circuit. The torque detector circuitcomputes the magnitude and the direction of the torque based on thevalue and the sign of the rectified signal. In this manner, theprocessor detects an applied torque based on voltages from the detectingcoils 10, 11. The computation of the difference performed by thedifferential circuit compensates for external noise such as temperaturechanges. Accordingly, the accuracy of torque detection is improved. Thetorque computing circuit detects the direction of the torque based onwhether the rectified signal has positive value or a negative valuerelative to a reference value (zero) and also detects the magnitude ofthe torque based on the magnitude of the rectified signal. The referencevalue is set such that it becomes zero when no torque acts on the shaft2.

This torque sensor 1 has the following characteristic features.

When the torque sensor 1 is operating, an alternating-current powersupply supplies alternating-current to the exciting coils 8, 9. Thesupplied current has a constant amplitude and frequency. The excitingcoils 8, 9 produce two magnetic circuits through the stator 7 and themagnetostrictive member 5. Since the bobbins B1, B2 are made of resin,the coefficient of thermal expansion of the resins B1, B2 is greaterthan that of the yoke 13 and the inner yoke 14. When the bobbins B1, B2are expanded by heat, the clearances C1, C2 accommodate the expansion ofthe bobbins B1, B2. Thus, the bobbins B1, B2 are not pressed against theouter yokes 12, 13 and the inner yoke 14, which eliminates undesiredstress acting on the bobbins B1, B2. As a result, the magneticcharacteristics of the exciting coils 8, 9 and the detecting coils 10,11 are maintained. The thermal characteristics of the torque sensor 1are maintained accordingly, which improves the accuracy of the torquedetection.

Further, the annular projection 15 engages with the annular recess 16,which facilitates positioning of the bobbins B1, B2 in the radialdirection. Therefore, the clearances C1, C2 are accurately formed aboutthe bobbins B1, B2.

The bobbins B1, B2 are firmly fixed to the surfaces 100, 101. Thus, whenthe torque sensor 1 vibrates, the bobbins B1, B2, or the coils 8 to 11,are not displaced. Thus, the magnetic characteristics of the coils 8 to11 are maintained. As a result, the detecting coils 10, 11 generateinduced electromotive force that accurately corresponds to the strain ofthe regions X, Y on the magnetostrictive member 5, or to torque actingon the shaft 2. Thus, the torque is accurately detected.

The torque sensor 1 of FIGS. 1, 2 and 6 does not have the retainers 37of the prior art torque sensor shown in FIG. 4, and the stator 7 isformed with three parts, namely, the outer yokes 12, 13 and the inneryoke 14. This construction reduces errors produced during assembly.Since the outer yokes 12, 13 and the inner yoke 14 are cylindrical, thecross section of the stator 7 is substantially perfectly round, and thestator 7 is therefore substantially coaxially assembled with themagnetostrictive member 5. Thus, the inductance of each detecting coil10, 11 is scarcely affected by the rotational position of the shaft 2.Accordingly, the reliability of the torque sensor 1 is improved.

Since the outer yokes 12, 13 have the same shapes, they areinterchangeable. Compared to the case where two different shaped outeryokes are used, the outer yokes 12, 13 reduce the manufacturing cost.

A torque sensor according to a second embodiment of the presentinvention will now be described with reference to FIG. 3.

The torque sensor of the second embodiment has a stator 7A shown in FIG.3. An annular groove 26 is formed in the inner wall of the stator 7A.The shape of the groove 26 is different from the shape of the groove 24shown in FIGS. 1 C2 and 2. A pair of bobbins BA is fixed to an inneryoke 21 by adhesive. Thus, an annular clearance C3 is defined betweenthe bobbin BA and the end wall of a holder portion 20 a of the firstouter yoke 20. The axial dimension of the clearance C3 is between 0.1 mmand 0.2 mm.

An annular projection 22 is formed on the end wall of the inner yoke 21,and an annular recess 23 is formed in the inner end wall of the bobbinBA. The projection 22 engages with the recess 23.

Adhesive is applied to the surfaces 10A, 101A of the projection 22 ofthe inner yoke 21. The recess 23 of the bobbin BA is engaged with theprojection 22. Then, the connector 21 is lightly press fitted into thefirst outer yoke 20 to form the stator 7A. The adhesive on the surfaces10A, 101A prevents the bobbin B1 from moving radially and axially.

In the embodiment of FIG. 3, a pair of bobbins BA are secured to thesides of the inner yoke 21. This structure allows electric connectorsfor drawing out the wires of the exciting coils 8, 9 and the detectingcoils 10, 11 to be located in the inner yoke 21. Further, the structureallows a common connector to be used to draw out wires of the coils 8 to11.

FIG. 7 illustrates a torque sensor according to a third embodiment. Inthe embodiments of FIGS. 1 to 3 and 6, the stators 7 and 7A are formedwith annular parts, which are separated in the axial direction of theshaft 2. The stator of FIG. 7 is formed with semi-cylindrical parts,which are separated by a plane that includes the axis of the shaft 2.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Particularly, it should beunderstood that the invention may be embodied in the following forms.

In the embodiments of FIGS. 1, 2, 3, 6 and 7, the numbers of the bobbinsB1, B2 and BA may be changed.

In the embodiments of FIGS. 1, 2, 3, 6 and 7, the shape and the numberof the parts forming the stators 7, 7A may be changed as long as thestators 7, 7A have a space for accommodating the bobbins B1, B2 and BA.Also, the stators 7, 7A may be directly coupled to the shaft 2 bybearings.

In the embodiments of FIGS. 1, 2, 3, 6 and 7, the inner yokes 14, 21 aresmaller in diameter than the outer yokes 12, 13 and 20. However, theinner yoke may be larger in diameter than the two outer yokes. Also, thegrooves for holding the bobbins may be entirely formed within the inneryoke, rather than being formed between the inner and outer yokes.

The present examples and embodiments are to be considered asillustrative and not restrictive and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

What is claimed is:
 1. A torque sensor for sensing torque applied to ashaft having a magnetostrictive member fixed to the shaft, wherein themagnetostrictive member is strained by the torque applied to the shaft,an exciting coil for generating flux running through themagnetostrictive member, a detecting coil for detecting the fluxvariation, the torque sensor comprising; a bobbin about which theexciting coil and the detecting coil are wound, a stator for supportingthe bobbin such that the coils surround the magnetostrictive member, andwherein the stator is rotatably supported relative to the shaft and isformed with a plurality of stator pieces, the stator having an annulargroove formed in an inner surface of the stator to accommodate thebobbin, the annular groove having a first end wall, a second end walland a circumferential wall and the bobbin having a first end surface, asecond end surface and a circumferential surface, the first end surfaceof the bobbin is fixed to the first end wall of the annular groove, anda first annular clearance is formed between the circumferential surfaceof the bobbin and the circumferential wall of the annular groove, and asecond annular clearance is formed between the second end surface of thebobbin and the second end wall of the annular groove.
 2. The torquesensor according to claim 1, wherein the stator includes an annularinner yoke having a first end and a second end, a first annular outeryoke and a second annular outer yoke, wherein the first outer yoke andsecond outer yoke are respectively coupled to the first end and thesecond end of the inner yoke, and wherein the annular groove is a firstannular groove defined between the first outer yoke and the inner yokeand a second annular groove is defined between the second outer yoke andthe inner yoke.
 3. The torque sensor according to claim 2, wherein astopper for preventing the bobbin from being axially and radiallydisplaced is formed between the first end surface and the first endwall.
 4. The torque sensor according to claim 3, wherein the stopperincludes a recess formed in the first end surface of the bobbin and aprojection formed on the first end wall of the annular groove, andwherein the projection engages with the recess.
 5. The torque sensoraccording to claim 4, wherein the recess is formed at the radially innerportion of the first end surface of the bobbin, and wherein theprojection is formed at the radially inner portion of the first end wallof the annular groove.
 6. The torque sensor according to claim 5,wherein the projection and the recess are annular.
 7. The torque sensoraccording to claim 2, wherein the bobbin is a first bobbin and is fixedto the first outer yoke, and the torque sensor includes a second bobbinfixed to the second outer yoke.
 8. The torque sensor according to claim7, wherein the first and second bobbins are fixed with adhesive.
 9. Thetorque sensor according to claim 2, wherein the bobbin is a first bobbinand is fixed to one end of the inner yoke, and the torque sensorincludes a second bobbin fixed to the other end of the inner yoke. 10.The torque sensor according to claim 9, wherein the first and secondbobbins are fixed with adhesive.
 11. The torque sensor according toclaim 2, wherein the outer yokes have substantially the same shape, andwherein the first and second yokes are symmetrically arranged withrespect to the inner yoke.
 12. The toque sensor according to claim 1,wherein the first annular clearance and the second annular clearance arebetween 0.1 mm and 0.2 mm.
 13. The torque sensor according to claim 1,wherein the cross-sectional shape of the inner surface of the stator issubstantially a complete circle.
 14. The torque sensor according toclaim 1, wherein the stator is rotatably supported on the shaft by abearing.
 15. The torque sensor according to claim 1, wherein themagnetostrictive member includes a first magnetostrictive region, inwhich flux is increased when torque acts on the shaft, and a secondmagnetostrictive region, in which flux is decreased when torque acts onthe shaft, wherein the exciting coil is a first exciting coil and thedetecting coil is a first detecting coil, and the first exciting coiland the first detection coil are located to correspond one of the firstand second magnetostrictive regions, and a second exciting coil and asecond detecting coil are located to correspond to the other of thefirst and second magnetostrictive regions, wherein each exciting coilproduces flux in the corresponding magnetostrictive region, wherein eachdetecting coil detects changes in flux in the correspondingmagnetostrictive region, and wherein the torque is detected based on thedifference between the voltages induced in the detecting coils.